Rhenium is an expensive and rare metal that has found a number of niche applications. The metal currently finds predominant use in, for example, petroleum-reforming catalysts (e.g., bimetallic Pt—Re compositions) for the production of high-octane hydrocarbons and in superalloys used in high-temperature turbine engine components. These two applications represent an estimated 20% and 60%, respectively, of the end use of rhenium. Rhenium is also known to improve the high-temperature strength properties of some nickel-based superalloys.
Rhenium has also been increasingly used as a promoter in ethylene oxide catalysts containing silver as the active metal on a solid support. These ethylene oxide catalysts are used on a large scale for the production of ethylene oxide, an important precursor to numerous commodity chemicals (e.g., ethylene glycol) used on a large scale. Ethylene oxidation catalysts typically contain up to about 0.5 wt % rhenium by weight of catalyst. Considering the scarcity and high price of rhenium, the amount of rhenium in these catalysts is substantial. A more detailed understanding of these rhenium-containing ethylene oxide catalysts can be found in, for example, U.S. Pat. Nos. 4,766,105, 4,808,738, 4,820,675, 5,364,826, 4,829,044, 5,418,202, 5,739,075, 5,545,603, 5,663,385, 5,739,075, 5,801,259, 5,929,259, 6,372,925, and 6,368,998.
According to the U.S. Geological Survey, Mineral Commodity, January 2008, the price of rhenium reached $5,000 per kilogram in January 2007, $7,000 per kilogram in April 2007, and $9,000 per kilogram in September 2007. In early 2008 the price of rhenium surpassed the $10,000 per kilogram mark.
Recent high demand for rhenium and the sharp increase in its cost makes it essential, as well as profitable, to recover rhenium. Accordingly, there has been much interest in finding improved methods for recovering this precious metal. See, for example, U.S. Pat. Nos. 2,967,757, 3,260,658, 3,348,942, 3,407,127, 3,458,277, 3,798,305, 3,862,292, 3,855,385, 3,733,388, 3,932,579, 4,185,078, 4,049,771, 3,244,475, 4,278,641, 4,521,381, 4,557,906, 4,572,823, 4,599,153, 4,599,223, and 7,166,145; U.S. Application Publication Nos. 2003/0119658, 2007/0203351, and 2007/0227903; foreign patent document GB 2 009 119A; as well as literature references Hydrometallurgy, Volume 78, Issues 3-4, August 2005, pages 166-171; Hydrometallurgy, Volume 85, Issue 1, January 2007, pages 17-23; and Ind. Eng. Chem. Res., Volume 38 (5), 1999, pages 1830-1836.
Most of the art cited above disclose the use of water either alone or in aqueous solutions for extracting rhenium from spent catalysts and other rhenium-containing sources. Particularly in the case of spent ethylene oxide catalysts, a major disadvantage in using water is the concomitant extraction of numerous other metal promoters along with rhenium. These other promoters typically include such elements as Li, Na, Cs, S, P, W, Ni, Hf, Ti, Zr, and/or B. Because of this, the resulting aqueous solution of rhenium will also be contaminated with varying amounts of these other promoters.
Since these other promoters need to be removed in order to recover pure rhenium, several additional steps are typically required to isolate rhenium. One common method for separating rhenium from these other elements is based on the selective adsorption of rhenium. Selective adsorption of rhenium is typically based on ion exchange or carbon adsorption. When adsorptive techniques such as these are used, there is required an additional step of leaching off the adsorbed rhenium before isolating the recovered rhenium. These additional steps increase the complexity and cost of the process while decreasing the efficiency of the process. Furthermore, as the number of steps in the recovery process increases, there is a greater loss of rhenium, which translates into a smaller recovery.
In addition, it is well-known and highly prevalent in the art not only to use aqueous solutions, but to use strong acids (e.g., the mineral acids, such as HCl, HNO3, H2SO4, aqua regia, and the like), to solubilize and/or process rhenium from a spent rhenium-containing source. As is well known, the corrosiveness of such acids is very high. Because of this, special equipment and handling procedures need to be employed. In addition, safety considerations become a major issue. Further, the use of such strong acids usually necessitates the use of a neutralizing base downstream from the acidification process once rhenium has been separated from other components. The neutralization process very often requires a strong or highly caustic base effective for neutralizing the strong acid. This further increases the need for specialized equipment and special safety measures. The neutralization process also produces more chemical waste. Since the waste must be properly disposed of, the production of this chemical waste becomes another significant financial liability.
From the above survey of the prior art, it is evident that there is a need for a new method for the recovery of rhenium that is simpler (e.g., requiring fewer steps), more efficient, and less costly. There would also be a particular advantage in such a process being selective for the removal of rhenium while in the presence of one or more other elements typically used as active catalyst metals or promoters in ethylene oxide catalysts.